Re: [PULL 1/2] hw: m68k: virt: Add compat machine for 6.1

[BALATON Zoltan]

On Tue, 9 Nov 2021, Daniel P. Berrangé wrote:
> On Tue, Nov 09, 2021 at 01:34:49PM +0100, BALATON Zoltan wrote:
> > On Tue, 9 Nov 2021, Laurent Vivier wrote:
> > > Add the missing machine type for m68k/virt
> > >
> > > Cc: qemu-stable@nongnu.org
> > > Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> > > Message-Id: <20211106194158.4068596-2-laurent@vivier.eu>
> > > Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> > > ---
> > > hw/m68k/virt.c | 9 ++++++++-
> > > 1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/m68k/virt.c b/hw/m68k/virt.c
> > > index 4e8bce5aa6f7..0d9e3f83c169 100644
> > > --- a/hw/m68k/virt.c
> > > +++ b/hw/m68k/virt.c
> > > @@ -304,7 +304,14 @@ type_init(virt_machine_register_types)
> > >     } \
> > >     type_init(machvirt_machine_##major##_##minor##_init);
> > >
> > > +static void virt_machine_6_1_options(MachineClass *mc)
> > > +{
> > > +}
> > > +DEFINE_VIRT_MACHINE(6, 1, true)
> > > +
> > > static void virt_machine_6_0_options(MachineClass *mc)
> > > {
> > > +    virt_machine_6_1_options(mc);
> > > +    compat_props_add(mc->compat_props, hw_compat_6_0, hw_compat_6_0_len);
> > > }
> > > -DEFINE_VIRT_MACHINE(6, 0, true)
> > > +DEFINE_VIRT_MACHINE(6, 0, false)
> >
> > I don't understand how these compat machines work but if these are empty and
> > essentially the same as the previous version why do we add a new version in
> > every release? Wouldn't it be enough to add new version when there was an
> > incompatible change? I mean, instead of listing machine and getting a lot of
> > virt-6.1, virt-6.0, virt-5.2,... or so, we'd only get versions that are
> > actually different such as virt-7.0, virt-5.2, virt-5.0 (maybe they are
> > called differently, just an example) with the versionless alias always
> > pointing to the latest. Then when QEMU is updated one can see if there was
> > any change so should update the VM or keep using the older versions. Or does
> > it work like that and I'm missing it completely?
>
> It doesn't work like that, and that's a good thing.
>
> The versioned machine types are for management applications that want
> to guarantee an ABI across hosts. When a mgmt app wants to set a new
> baseline for their QEMU machine types, it is way clearer if every
> versioned machine type across all target arches supports the same
> versions, regardless of whether there were any changes or not.
>
> ie if an app wants to set QEMU 6.1.0 as the baseline, they want
> to be able to set  virt-6.1 for aarch64, for mips, for riscv,
> instead of having to figure out which versions exists for each
> arch

With a bit more logic the management app could easily implement this 
either by dereferencing the alias when creating the VM, say virt is an 
alias of virt-6,0 in QEMU 6.2 then put virt-6.0 in the VM config which 
then will work on anyting newer than QEMU 6.0. Or record the QEMU version 
in VM config when creating the VM saying this needs at least QEMU 6.1 then 
if there's no virt-6.1 or virt points to something newer then go back to 
the biggest version less than 6.1 (this might be more difficult due to 
changes in QEMU versioning but if we assume that at least they are 
increasing numbers it should be possible to find the largest version less 
than 6.1 for example). That way when a user types -machine help they won't 
get a large list of useless versioned machines that are mostly the same 
anyway. This looks like a case that a lazy management sofrware pollutes 
the human interface while it could handle it itself. But that's just my 
opinion, I don't use these too much so don't care too much either. It just 
looked like something that is putting a burden both on users and 
debelopers (having to add more compatiblity stuff to code that's already 
heavy on boilerplate) for something that could be handled within the 
management software. But maybe it's too late to change at this point.

Regards,
BALATON Zoltan